The obesity pandemic brings with it multiple attendant metabolic comorbidities, including Type 2 Diabetes (T2D) and Non-Alcoholic Fatty Liver Disease (NAFLD), which are likely to be the defining health and public policy issues of the 21st century. NAFLD is the number one cause for chronic liver disease, with prevalence approaching 30% in certain populations, but may in fact be considered a pre-disease state for Non-Alcoholic Steatohepatitis (NASH). NASH, which is defined by hepatocyte damage and associated inflammation and fibrosis, predisposes to cirrhosis and hepatocellular cancer, and is the fastest-growing reason for liver transplantation. NASH has no approved pharmacotherapy - as the prevalence of obesity-related NASH continues to rise, and available livers for transplantation remain limiting, this unmet need grows more urgent. Notch is a highly conserved family of proteins critical for cell fate decision-making, but less is known about Notch action in mature tissue. Our initial characterization demonstrated that Notch signaling is present at low levels in normal physiologic conditions, but increases markedly in livers from diet-induced or genetic mouse models of obesity, or obese patients with T2D or NAFLD. Surprisingly, hepatic Notch activity is higher still in patients with NASH, or in dietary mouse models which approximate human NASH. At the level of individual patient, or in mouse models of steatohepatitis, markers of Notch activity show significant positive correlations with biochemical (ALT levels) and pathologic (NAFLD Activity Score) markers of NASH. These results suggest that Notch activity in liver may be either a biomarker of hepatocyte damage, or may be causative to NASH. In this application, we will examine the mechanisms underlying activation of hepatic Notch signaling in NASH and its potential as a novel therapeutic target for obesity-induced NASH. In Aim 1, we will determine whether genetic or pharmacologic inhibition of hepatocyte Notch signaling prevents steatohepatitis and fibrosis induced by high-fat/high-carb feeding in mice. In Aim 2, we will determine which cell, and by which ligand, propagates the Notch signal in steatohepatitis, and determine whether we can exploit this knowledge to design safe and specific Notch inhibitors for treatment of NASH. Achieving the goals of this application will identify the underlying mechanism of increased Notch signaling in obesity-induced liver disease, as well as potentially repurpose existing Notch inhibitors as novel and unique pharmacotherapy for NASH.